1. Field of the Invention
The present invention relates to a photometry device for a camera, especially for a compact camera or a lens-fitted photo film unit, that measures subject brightness with reliability at a low operation voltage.
2. Background Arts
As a kind of handy economy camera, lens-fitted photo film units have been widely known. As being preloaded with a roll of unexposed photo film, the lens-fitted photo film unit makes it possible for anyone to enjoy photography anytime anywhere. For the sake of simplifying the structure and thus cutting the manufacture cost, the exposure value is fixed in most kinds of film units. With such a fixed exposure value, however, the exposure amount can get lower or higher than the latitude of the preloaded photo film under some photographic conditions.
In order to control exposure value automatically according to the subject brightness, the film unit must be provided with a photometry device for the subject brightness. In many of conventional cameras with an automatic exposure control device, a CdS (cadmium sulfide) cell is mounted as a photoconductive element or a photo sensor on a front side as the photometry device. Since the CdS photo sensor changes its resistance with the amount of light received thereon, it is possible to detect the subject brightness as the resistance value.
As a power source for driving the photometry device with the CdS photo sensor, a battery with a terminal voltage of 3V or more has generally been used. Since the film unit usually uses a battery having a terminal voltage of 1.5 V for supplying its built-in flash device, it would be necessary to provide the film unit with a second battery for mounting the conventional photometry device in the film unit. The second battery would increase the size and manufacture cost of the film unit. Beside this problem, sensitivity characteristics of the CdS photo sensor can vary between individual products, and also depending upon temperature and power source voltage.
In view of the foregoing, an object of the present invention is to provide a photometry device for automatic exposure control of a camera, that operates stably even at a low power source voltage without being affected by environmental conditions.
According to the present invention, a photometry device provided in a camera, for detecting whether a subject brightness is less than or not less than a reference level, comprises a photoconductive element located to receive light from a subject, a first end of the photoconductive element being connected to a first electrode of a power source; a resistor whose first end is set at the same potential as the first end of the photoconductive element; a first transistor connected at its base to a second end of the photoconductive element, at its collector to a second end of the resistor and at its emitter to a second electrode of the power source; a second transistor connected at its base to the second end of the resistor, at its collector to the second end of the photoconductive element, and at its emitter to the second electrode of the power source; and a switching transistor which is turned on in response to a predetermined one of the first and second transistors being turned on, wherein the first and second transistors are turned on and off in contradiction to each other depending upon whether a current flowing through the photoconductive element is less than or not less than a current flowing through the resistor, so the switching condition of the switching transistor is used for detecting whether the subject brightness is less than or not less than the reference level.
The photometry device having the above configuration operates stably even when the power source voltage is low.
The first and second transistors are preferably the same type having the same properties, and a resistance between the base of the second transistor and the first end of the resistor is set equal to a resistance that is generated between the first end of the photoconductive element and the base of the first transistor when the subject brightness is equal to the reference level.
Thereby, the photometry is not affected by the temperature variations or other environmental conditions.
To use the photometry device as a member of an automatic exposure control device, the switching transistor may be connected to a stop changing device for changing stop size of the camera.
According to a preferred embodiment, current from the power source is supplied through a photometry switch to the photoconductive element and the resistor. The photometry switch is preferably turned on in response to a shutter release operation on the camera. Also a capacitor is connected in parallel to the power source and charged by the current from the power source while the photometry switch is on. After the photometry switch is turned off, the photoconductive element and the resistor are supplied with current discharged from the capacitor. Thereby, the first and second transistors keep their switching conditions for a time enough for the camera to complete an exposure in response to the shutter release operation.
Where the power source is used for supplying a flash circuit of the camera, the photometry device further comprises a device for stopping the flash circuit from charging in response to the photometry switch being turned on. Thereby, the power source voltage would not drop while the photometry device is activated, so the photometry device operates stably.